轴对称降落伞小迎角稳定下降时流场特性

Influence of Low Angle of Attack on the Flowfield Characteristics of an Axisymmetric Parachute in Terminal Descent

根据降落伞的特点，通过伞衣零厚度假设、伞轴对称假设和流场定常假设，建立轴对称降落伞的流体力学计算模型。在0°～5°的小迎角范围内，求解RNG（Renormalization Group）k-ε湍流模型下的N-S方程组，获得与有关单位试验相吻合的数值模拟结果。分析发现0°迎角时轴对称降落伞截面流场中鞍点与鞍点直接连接的状态是不稳定的。随着降落伞迎角增大，奇点类型和奇点间的连接方式将发生改变，降落伞物面附近的涡环里将出现极限环，截面流场中将出现新的鞍点和结点。迎角继续增大，极限环的面积将会减小直至消失，结点将附着在物面上，成为半结点。结果表明小迎角范围内降落伞截面流场的拓扑结构符合拓扑规律。文章揭示了小迎角范围内轴对称降落伞稳定下降时流场特性的演变规律，为进一步研究降落伞流场的流动机理和流固耦合问题打下基础。

According to the characteristics of parachute, based on 0-thickness assumption of canopy, axisymmetric assumption of parachute, and steady assumption of flowfield, a fluid dynamics computational model was established. By solving RNG （Renormalization Group） k-epsilon turbulence N-S Equations over low angle of attack range of 0-5°, the numerical simulation result was in good agreement with the relative experiments. It was found that the direct connection of saddle point in the axisymmetric parachute sectional flowfield of 0° angle of attack was unstable. With the increase of angle of attack, the types and connection models of singular point will be changed. Limit cycles will appear in the vortex ring near the parachute surface. Also new saddle points and a nodal point will appear in the section flowfield. With the further rising of angle of attack, the area of limit cycles will decrease until limit cycles disappeared. The nodal point will change into a half-node and attach to the parachute surface. The results indicate that the topological structures of sectional flowfield of an axisymmetric parachute at low angle of attack accord with topological rules. The gained evolvement rules of the flowfield characteristics of an axisymmetric parachute in terminal descent here may lay foundation for further investigating parachute flow mechanism and fluid-structure coupling.